Infants are routinely given a series of vaccinations within the first few months of life to confer protection against potentially life-threatening bacterial and viral diseases. Some antigens, especially the carbohydrate antigens of bacteria, fail to induce a protective immune response in infants. These antigens must be administered with a protein carrier to stimulate an immune response. For example, an antigen such as the capsular polysaccharide of Haemophilus influenzae type b (Hib), the bacteria primarily responsible for bacterial meningitis, must be conjugated to a carrier protein for successful induction of an antibody response in infants.
A variety of chemical linkages have been used to prepare polysaccharide-protein conjugates. However, the coupling methods employed are time-consuming and result in linkages, such as amido linkages, that cause excessive crosslinking or detrimental modifications to the antigen, which are undesirable in human vaccines. For example, polysaccharides may be coupled to proteins using reductive amination as described in U.S. Pat. No. 4,356,170 to Jennings et al., entitled "Immunogenic Polysaccharide-Protein Conjugates". Conjugation is achieved by oxidizing the polysaccharide with an oxidizing agent, coupling the oxidized polysaccharide to a protein, and reducing the bond with a reducing agent for stability. This process requires lengthy dialysis and incubation steps and chromatography purification and may take up to two to three weeks for completion.
What is needed is a rapid, inexpensive polysaccharide-protein conjugation method that results in a stable conjugate capable of conferring an immune response that renders protection against microbial infections, especially in human infants.